Conventionally, an ultrasonic actuator disclosed in Patent Document 1 has been known as an ultrasonic actuator including an actuator body performing a plurality of vibrations including a bending vibration.
The ultrasonic actuator of Patent Literature 1 includes an actuator body constituted of a piezoelectric element, and a driver element attached to the actuator body.
The actuator body is constituted of a flat plate-shaped piezoelectric element having a longitudinal direction. In the actuator body, a longitudinal vibration (i.e., a so-called expansion/contraction vibration) in the longitudinal direction of the piezoelectric element and a bending vibration in a lateral direction of the piezoelectric element are induced harmonically by respectively applying AC voltages having different phases to two electrode pairs, each of which includes diagonally aligned two electrodes. As a result, the driver element makes an orbit motion, more specifically, an elliptical motion, in a plane including the longitudinal direction and the lateral direction of the piezoelectric element.
The driver element is substantially in the shape of a hemisphere. Two driver elements are arranged on a long side surface of the actuator body. The long side surface is a surface which is normal to a vibration direction of the bending vibration of the actuator body, and is bent and deformed in response to the bending vibration of the actuator body. The two driver elements are arranged at positions on the long side surface corresponding to antinodes of the bending vibration where bending displacement is the maximum.
In the ultrasonic actuator thus configured, the driver elements are arranged in contact with the drive target. When the ultrasonic actuator is driven in this state, a driving force of the actuator body is transmitted to the drive target through the driver elements making the orbit motion as described above, and therefore the drive target is driven. At this time, the actuator body is biased toward the drive target, and is configured so that a friction force between the driver elements and the drive target is increased to efficiently transmit the driving force of the actuator body.    Patent Document 1: Published Japanese Patent Application No. 2004-304963